Every so often at power-up, solenoid fuse F2 (2.5 A slow-blow) blows. When it happens, all the playfield coils fire at once; the flippers, on their own circuit, keep working. Coils and wiring check out.

Solenoid fuse F2 blows at power-up as every coil fires: CPU blanking circuit

Cause: The blanking circuit on the CPU board, which holds the drivers off while the board starts up, was failing intermittently.

Checks

• Watch the playfield at power-up: coils all kicking at once points to blanking, not a shorted coil.

Fix

• Replace the blanking circuit parts on the CPU board: the 555 timer, its 1 uF capacitor and the 2N4403 transistor.

Systems

• Williams System 7

Games

• Black Knight

Related tips

• Blanking stays low: timer, C31 and Q5
• Blanking LED wrong, or all solenoids pulse and blow a fuse at power-on
• Displays flash bright and speakers noisy, game does nothing
• Blanking circuit parts: C32/C84, IC7, Q5/Q1 and IC23
• Re-jumper a System 7 CPU from Defender/Hyperball/Star Light to a standard game
• Fliptronics flipper coil wired wrongly
• STNG: under-playfield diverter coils lock on during play
• CPU flipper enable relay RY1

Source: Pinball Repair help group!! (Facebook group)